Chromium-molybdenum (CrMo) and chromium-molybdenum-vanadium (CrMoV) low-alloy steels have found wide use for components of steam turbines and other power plant applications. These alloys have been selected in part on the basis of creep and fatigue properties due to the severe operating conditions of steam turbine components. Although steam turbine components made from CrMo and CrMoV steels exhibit a long service life, it is possible that wear, erosion, corrosion, shock, fatigue and/or overstress will occur, necessitating repair or replacement of the component. In the past, repairs of CrMo and CrMoV components have often been performed by welding, in which the damaged portion of the component is removed and a steel weldment is built up in its place. After repair, the component has traditionally undergone a post-weld heat treatment (PWHT) in order to relieve stresses induced by the weld repair process and to temper the hardened weld heat-affected zone (HAZ), thereby developing properties similar to that of the original alloy.
Comparative hardness data for as-welded and post-weld heat treated CrMoV alloy is shown in FIG. 2. Hardness is measured within the HAZ of the weld region, where the chemical composition of the base alloy is generally unchanged by the welding operation but considerable microstructural change has occurred as a result of the severe thermal cycle that takes place during welding. One effect of this microstructural change is that considerable hardening occurs within the HAZ. In FIG. 2, hardness is indicated within the HAZ relative to the distance from the fusion line, defined here as the interface between the weld repair material and the base material of the component. Because weld-induced hardness is detrimental to creep and fatigue properties, FIG. 2 evidences that post-weld heat treatment is needed to promote these desirable properties in a weld-repaired component.
In the past, post-weld heat treatment has entailed heating the entire component to a temperature below the critical temperature "A.sub.1 " of the alloy, which is defined in the art as the lower limit of the face-centered cubic lattice crystallographic structure (austenite) of the iron-carbon equilibrium diagram. A drawback to this process is the cost of the heat treatment operation, the time involved to perform the operation, and the possibility of distorting the component. These drawbacks are particularly problematic in the repair of high-temperature components that have heightened creep resistance requirements, such as the CrMoV turbine lower shell of a steam turbine. Removal of these components for post-weld heat treatment is costly and time-consuming due to piping connections that must be cut and rewelded, realignment of the turbine after repair, etc. However, repair of such components without a post-weld heat treatment generally yields unsuitable results, and attempts at in-situ post-weld heat treatment have a tendency to distort the shell.
Consequently, weld repair techniques have been proposed using filler materials that do not require post-weld heat treatment. However, such techniques are generally limited to temporary repairs and some noncritical applications, because weld repairs that undergo post-weld heat treatment generally exhibit superior properties. Other alternatives for making weld repairs without post-weld heat treatment include various temperbead welding techniques, in which a carefully controlled welding sequence provides some degree of tempering by superimposing a suitable temperature region of the HAZ from the weld bead being deposited on a hard portion of a HAZ from a previously deposited weld bead. Such techniques have been performed in an attempt to provide beneficial softening of hard metallurgical structures in the HAZ. However, when applied to some CrMo and CrMoV alloy steel components, undesirable HAZ hardening has been found, as evidenced by the data scatterband for temperbead techniques shown in FIG. 2.
Accordingly, what is needed is a reliable weld repair technique for low-alloy steels, such as CrMo and CrMoV alloys used to form steam turbine components, that produces a long-lasting weld repair without resorting to post-weld heat treatment of the repaired component and without causing distortion of the component.